An aircraft (A/C) in motion is usefully described in terms of energy content. The energy content of an airborne A/C is primarily determined by the altitude of the A/C, the airspeed at which the A/C is traveling, and gross A/C weight. Generally stated, a pilot can selectively increase the energy content of an A/C through gains in airspeed or altitude achieved by applying additional thrust. Conversely, a pilot can bring about a controlled decrease in A/C energy content over time by hastening the rate at which the A/C energy content dissipates. This may be accomplished by increasing the drag coefficient of the A/C by, for example, altering the angle of attack of the A/C or by deploying one or more drag devices, such as flaps, slats, and airbrakes. Other factors, which may reside outside of a pilot's control, can also influence the energy content of an A/C, such as shifts in wind speed and direction.
Proper management of A/C energy content is particularly vital during approach and landing. When approaching an airport or other airfield for landing, a fixed wing A/C ideally arrives at a predetermined distance ahead of its destination runway at an airspeed affording the aircrew sufficient opportunity to configure the A/C for landing (hereafter, the “configuration distance”). If the A/C reaches the configuration distance in an under-energy state (that is, with an excessively low airspeed or Height Above Threshold (HAT)), additional thrust may be required to return the A/C to the acceptable energy state for landing. The application of such thrust, which is otherwise unneeded, results in decreased fuel efficiency, increased noise and chemical emissions, exacerbated component wear, higher operational costs, and other such undesired effects. Conversely, if an A/C reaches the configuration distance in an over-energy state (that is, with an excessive airspeed or HAT), the aircrew may be forced to abort the current landing attempt and initiate go-around. If, instead, touching down on the runway in an over-energy state, the A/C may be unable to adequately dissipate the remainder of its energy content during rollout and a runway excursion may occur. Both of these situations add undesired cost and delay to aircraft operation and can potentially contribute to air traffic congestion, detract from passenger comfort, and have other negative consequences.
Although the situations above are desirably avoided, pilot mismanagement of A/C energy content during approach and landing continues to occur for multiple reasons. First, it should be recognized that approach and landing are amongst the most dynamic and demanding phases of flight. Second, at any juncture after assigning a multi-leg approach route to a particular A/C, Air Traffic Control (ATC) may clear the A/C to depart from the assigned route and instead proceed directly to the runway. In this instance, a pilot is suddenly tasked with ascertaining whether departure from the assigned multi-leg approach route will bring the A/C to the configuration distance in an acceptable energy state. Depending upon various factors, this can be difficult for a pilot to accurately ascertain even when presented with an ideal vertical descent profile, as calculated by a Flight Management System (FMS) and graphically represented on a Vertical Situation Display (VSD). If unsure as to the ramifications of accepting the ATC-proposed direct approach, a pilot may simply decline the direct approach option. This potentially results in a lost opportunity for improved operational efficiency for the aircraft and increased traffic throughput for the airfield. Alternatively, the pilot may accept the ATC-proposed direct approach. In so doing, however, a pilot risks mentally miscalculating the energy requirements of the direct approach relative to the current energy content of the A/C and/or otherwise risks mismanagement of the energy content of the A/C when carrying-out the direct approach.